Ironoxide -
I have seen and suffered from this exact issue before. I need you to look at something to tell if the diagnosis I have in my mind is at play. If so, the fix is a lot less mechanical and involved than you might think:
Look along the area where the barrel and frizzen meet. Is there a smooth scuff on the barrel's finish right there? Ignore any shark gouges or marks that may have been caused by accidentally seating the flint too far over and catching the barrel. I'm talking about something that looks like longer term burnishing.
If that is the case, you have a situation where the lock inlet was cut too deep. When this happens - and it doesn't take much of an overcut, particularly if you really tighten down the lock screws - the frizzen will get pulled too tight to the barrel and actually rub on it, causing the noticeable difficulty in opening it (and the partial openings and broken flints you are witnessing).
To test this...
1) Check to ensure that the gun is not loaded.
2) Loosen the lock screw opposite the frizzen ever so slightly
3) Physically push the lock screws themselves to assure that the entire lock assembly and the frizzen itself is now assuredly away from the barrel a *tiny* bit. We're talking just a hint of daylight between the frizzen and the barrel - perhaps 1/64".
4) Manually cycle the frizzen with your hand and see if the resistance has diminished. If so, you've likely confirmed the issue.
5) To completely confirm the diagnosis, point the gun in a safe direction and test the entire striking sequence by cocking and discharging the lock.
If the lock sparks as it should, and you don't get the frizzen hangup/broken flint problem, you now know the cause. In minor cases, you can simply leave the lock screw just a little less cinched than you normally do and the gun will run fine. If the over-inletting is more significant, you'll want to get some thin cork and cut some small pieces to put along points where lock mates to the stock to create a small lift point and more assertively remedy the issue. Test a few points and when you find it, glue the cork in place.
Try this test and please let us know how it goes. Hopefully, it will get you rolling in the right direction.